The art of surveying involves the determination of unknown positions, surfaces or volumes of objects using measurements of angles and distances. In order to make these measurements, a surveying instrument frequently comprises an electronic distance measurement unit (EDM unit) which may be integrated in a theodolite, thereby forming a so-called total station. A total station combines electronic, optical and computer techniques and is furthermore provided with a computer or control unit with writable information for controlling the measurements to be performed and for storing data obtained during the measurements. A typical total station comprises a base with tribrach mounted on a tripod, an alidade mounted on the base for rotation about a vertical axis and a center unit mounted on the alidade for rotation about a horizontal axis. The center unit of the total station comprises a telescope and optical elements for aiming at a target. In particular, the center unit is provided with an EDM unit which operates generally in the direction of the optical axis of the center unit, i.e. along the line of sight. In, for example, WO 2004/057269 by the same applicant, such a total station is described in more detail. Preferably, the total station calculates the position of a target in a coordinate system, e.g. a fixed ground-based coordinate system. However, as the total station measures the position of the target relative to its own position, the position (or location) of the total station in the fixed ground-based coordinate system needs to be known.
According to a first alternative, the position of the total station in a fixed ground-based coordinate system may be determined by a method called “free stationing”. In this method, the surveying instrument is placed at an arbitrarily chosen location in the coordinate system (e.g. at a worksite). First, a leveling procedure is performed such that the total station is not tilted when mounted on the tripod. In particular, it is determined whether the vertical rotational axis (nadir) of the total station is aligned with a vertically plumbed axis, i.e. the nadir of the instrument extends vertically. Then, the directions, i.e. the vertical and horizontal angles, and optionally the distances to two (or more) targets placed at two (or more) known points of the ground-based coordinate system are measured. These known points may also be called control points. The control points of the ground-based coordinate system correspond to reference points or reference marks located at the ground level, and the coordinates of these control points in the ground-based coordinate system are known. Once the directions to these two (or more) control points are obtained, the location of the total station, i.e. the location coordinates of the total station, in the ground-based coordinate system can be determined. A drawback of free stationing is that it is difficult to retrieve the location at which the total station is initially positioned if the total station needs to be moved to another position and then moved back to the initial position for e.g. acquisition of further measurements. Marking on the ground of the location at which a total station is positioned is difficult to achieve with high accuracy.
According to another alternative, the total station may be directly positioned above a control point of the ground-based coordinate system. However, an accurate positioning of the total station above such a control point is a quite tedious procedure. First, the tripod is approximately set over the control point and roughly leveled. The total station is then mounted on the tripod and centered above the control point by looking through the eye piece of an optical plummet. The eye piece provides a view with a center mark that allows centering of the total station directly over the control point. The final centering is performed according to an iterative procedure during which a surveyor slides the total station over the tripod (e.g. by moving the tribrach) until the total station is centered over the control point, i.e. with the center mark seen in the eye piece over the control point. Once the total station is centered and leveled, the total station is firmly attached to the tripod. At this stage, the coordinates (x, y) of the total station in the ground-based coordinate system are known and correspond to the coordinates of the control point. However, a complete localization of the total station requires also a determination of the height at which the total station is located and an orientation of the direction at which the total station is pointing. The height is normally determined using a measuring tape or a similar device; however, the accuracy of such a measurement is usually limited, thereby decreasing the accuracy of the measurements later made by the surveying instrument. The orientation of the total station is achieved by pointing the center unit of the surveying instrument at another control point (having a known position) located at a remote distance from the total station and determining the horizontal angle of the total station to this control point.
In summary, the prior art methods described above are tedious, of limited accuracy, time consuming and not user friendly.
Thus, there is a need for providing new methods and systems that would overcome these problems.